Quasi-dispersionless optical fiber transmission, dispersion compensation and optical clock

ABSTRACT

A method of transmitting optical pulses in a transmission media includes separating a coherent source optical pulse into a plurality of mutually coherent pulses, and producing a series of mutually coherent optical pulses from the plurality of pulses. The series is transmitted through the media, and the pulses of the series are received at a distant region of the media. The series of pulses is adapted to interfere and form a packet whose width is narrower than the width of any pulse of the series at the distant region.

This application claims the benefit of U.S. Provisional Application No.60/117,146, filed Jan. 25, 1999.

This invention was made with government support under Contract NumberF19628-95-C-0002 awarded by the Air Force. The government has certainrights in the invention.

BACKGROUND OF THE INVENTION

This invention relates generally to transmitting optical signals, andmore particularly, to reducing pulse broadening in optical fibers and tooptical clocks.

FIG. 1 shows that an initial optical pulse 2 becomes a broader pulse 3after traveling through an optical fiber 4. The broadening of the pulse2 results from dispersion. One reason for dispersion is the variation ofa fiber's index of refraction with wavelength. The index of refractionvariations make longer and shorter wavelength components of the pulse 2travel at different speeds in the optical fiber 4. After travelingthrough a certain length of the optical fiber 4, the speed variationsproduce the broader pulse 3. Another reason for dispersion is waveguidedispersion, which is induced by the geometric configuration of the fiber4.

Pulse broadening can affect the quality of digital data transmission inthe optical fiber 4. Digital data is transmitted as a series of opticalpulses. Each temporal interval for a source pulse represents one binarybit. The binary states “1” and “0” correspond to the presence andabsence of a pulse, respectively. As pulses broaden and overlap, areceiver may not be able to determine whether a pulse is present in aparticular time interval or whether a detected optical signal is thetail of a previous or subsequent pulse. Inserting an amplifier 5 intothe optical fiber 4 can aid to reduce receiver errors due to propagationweakening of pulse intensities. But, the amplifier 5 does not aid toreduce receiver errors caused by the dispersion generated pulsebroadening and overlap.

Present optical fiber communications use optical pulses havingwavelengths of about 1.5 microns, because erbium-doped fibers canprovide quality optical amplification at 1.5 microns. Unfortunately,many older optical fibers produce significant chromatic dispersion inoptical signals at 1.5 microns. This chromatic dispersion producessignificant pulse broadening, which limits transmission wavelengths anddistances in contemporary optical networks.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method for transmitting opticalpulses in a transmission medium. The method includes separating acoherent source optical pulse into a plurality of mutually coherentpulses, and producing a series from the plurality of pulses. The seriesis transmitted through the medium, and the pulses of the series arereceived at a distant region of the medium. The series of pulses isadapted to interfere and form a packet whose width is narrower than thewidth of any pulse of the series at the distant region of the medium.

In preferred embodiments, the method further includes dividing thesource pulse into a plurality of pulses directed into differentdirections, sending the pulse directed in each direction through aseparate optical waveguide to produce a delayed output pulse, andrecombining the output pulses to produce the series. Each output pulsehas a different delay.

In a second aspect, the invention provides an apparatus for transmittingan optical signal in an optical fiber. The apparatus includes an opticalbeam splitter to split a source light signal into a plurality ofseparated, mutually coherent light signals, an optical train to producea series of mutually coherent, outgoing light signals from the separatedsignals, and a combiner. Each outgoing light signal has a different timedelay. The combiner receives the outgoing light signals from each ofsaid conduits and is adapted to redirect the received light signals intoan optical fiber.

Embodiments of the invention provide methods and apparatus forquasi-dispersionless optical communications links. Some embodimentscompensate dispersion induced pulse broadening in optical fibers. Thereduced pulse broadening enables higher data rate transmission in longoptical fibers. Other embodiments compensate dispersion induced pulsebroadening occuring in free space propagation of optical signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the invention will beapparent from the following description taken together with the drawingsin which:

FIG. 1 illustrates pulse broadening in a prior art optical fiber;

FIG. 2 shows a system, which uses interference to reduce pulsebroadening in an optical fiber;

FIG. 3 illustrates diffraction broadening caused by a wide slit;

FIG. 4A illustrates multi-slit interference;

FIG. 4B illustrates interference from a Fresnel zone plate;

FIG. 5 shows an optical transmitter for use in the system of FIG. 2;

FIG. 6 shows an optical pulse splitter for use in the transmitter ofFIG. 5;

FIG. 7 is a flow chart illustrating a method of transmitting an opticalpulse in the system of FIG. 2;

FIG. 8 shows a non-uniformly spaced series of time-delayed coherentpulses to use in the system of FIG. 2;

FIG. 9 shows the packet produced by the series of pulses of FIG. 8 aftertraveling through an optical fiber;

FIG. 10 shows another pulse splitter, which uses a circulator to producethe series of pulses in FIGS. 2 and 8;

FIGS. 11A-11B show other pulse splitters, which use birefringent devicesto produce the equally spaced pulses in FIG. 2;

FIGS. 12A-12D show other pulse splitters, which use birefringent devicesto make the pulses shown in FIG. 2;

FIG. 13 illustrates another pulse splitter, which uses a series of 2×2fiber couplers to produce the equally spaced series of pulses in FIG. 2;

FIG. 14A shows an optical transmission system in which a pulse splitteris located in the transmitter;

FIG. 14B shows an optical transmission system in which a pulse splitteris located in the receiver;

FIG. 14C shows a transmission system, which uses pulse splitters andoptical amplifiers at intermediate locations along the transmissionfiber 52;

FIG. 15A shows a transmitter, which interleaves mutually incoherentpulses to reduce interference between pulses for different data bits;

FIG. 15B shows a regenerator or receiver, which uses a NOLM as a filter;and

FIGS. 16A-16C illustrate a high frequency optical clock, which employs apulse splitter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 schematically illustrates a system 50, which uses interference toreduce pulse broadening in an optical fiber 52. From each source pulseto be transmitted, transmitter 54 produces a series of mutually coherentpulses 56-59 and sends the mutually coherent pulses 56-59 into the.optical fiber 52. Each pulse 57-59 has a nonzero time delay with respectto the preceding pulse 56-58. The pulses 56-59 broaden and overlap dueto dispersion as they move down the fiber 52. For example, pulses 57, 58broaden to become overlapping pulses 60, 61 after a certain propagationtime. A receiver 65 receives the coherent broadened pulses.

At the distant receiver 65, pulse-overlap and interference produces aninterference pattern 64. The interference pattern 64 has narrow maxima66-68. The pattern 64 is similar to an interference pattern produced bya coherent light beam after passing through a multiple slit aperture.

To better appreciate the pattern 64 at the receiver 65 of FIG. 2, it isuseful to recall how single and multiple slit apertures produce opticalinterference.

FIG. 3 shows an intensity pattern 20, which a coherent light beam 22makes on a screen 24 located behind a wide slit 26. If the slit 26 isnot too wide, diffraction broadens the intensity pattern 20 to beyondthe width of the slit 26.

FIG. 4A shows an intensity pattern 28, which the coherent light beam 22produces on the screen 24 when located behind multiple narrow slits30-34. The intensity pattern 28 has maxima 36-38 and minima 42-44 due tointerference between light from the different slits 30-34. The lightfrom each slit 30-34 follows a different optical path to reach thescreen 24 and thus, has a different phase at the screen 24 than lightfrom the other slits 30-34. The phase differences cause the light tointerfere producing the central maximum 36, which is much narrower thanthe diffraction-widened pattern 20 from the wide slit 26 of FIG. 3. Theenvelope 40 of the intensity pattern 28 matches the pattern 20 from thewide slit 26 of FIG. 3.

The multiple slits 30-34 produce maxima 36-38 that become narrower asthe density of the slits 30-34 increases. Uniform spacings of the slits30-34 produce both the central maximum 36 and the secondary maxima 37,38 on each side.

FIG. 4B shows a interference pattern produced by a Fresnel zone plate46. The zone plate 46 interferes light from a coherent incoming beam 47to produce the interference pattern at the focal plane 48. Theinterference pattern consists of a well focused spot 49 withoutsubstantial secondary maxima, e.g., the maxima 37, 38 of FIG. 4A. Theabsence of secondary maxima is due to the non-uniform spacings of thetransmission rings 51 of the zone plate 46. The successive transmissionrings 51 have radii related by irrational ratios, i.e., the radii are R,{square root over ( )}2R, {square root over ( )}3R, {square root over ()}4R, etc. These non-uniform spaced rings 51 produce interference, whichresults in a single spot 49 on the focal plane 48.

Referring again to FIG. 4A, an interference pattern with single maximacan also occur if the ratios of the distances between each slit 31-34and the first slit 30 are irrational numbers. For example, if thedistance L_(N) of the Nth slit 31-34 from the first slit 30 satisfiesL_(N)=A(N)^(½), the secondary maxima 37, 38 become very small ordisappear. The irrationality of the slit spacings correspond toirrational phase differences from light received from the differentslits at the secondary maxima 37-38. Such phase differences causedestructive interference.

Referring again to FIG. 2, the interference pattern 64 in the receiver65 comes from the time delays in the series of original coherent pulses56-59. The interference pattern 64 has a narrower central maxima 67 thanthe envelope 70 that would result from sending a single pulse down thefiber 52.

FIG. 5 illustrates one embodiment 72 of the optical transmitter 54 shownin FIG. 2. A laser 74 produces a monochromatic light beam 76, which ischopped to a sequence of source pulses by a programmable high-speedshutter 77. The source pulses carry the binary data sequence to betransmitted to the receiver 65. Each source pulse enters a pulsesplitter 79, which produces a series of N delayed and coherent pulsesfrom the source pulse and sends the series of pulses to the opticalfiber 52.

The illustrated pulse splitter 79 uses a 1×N beam splitter 80, e.g., a1×N fiber coupler, to produce N mutually coherent pulses from eachsource pulse. The 1×N beam splitter 80 has an optical output along eachof N directions, and each output couples to an optical waveguideP₁-P_(N), e.g., optical fibers. Each optical waveguide P₁-P_(N) has anoptical length measured to produce one of the temporal delays of theseries of pulses 56-59 of FIG. 2. The optical waveguides P₁-P_(N) coupleto an inverted 1×N beam splitter 82 that recombines the delayed pulsesto produce the series of pulses 56-59 shown in FIG. 2. The pulsesplitter 79 may also include optical amplifiers (not shown) either inthe separate waveguides P₁-P_(N) or at its output.

FIG. 6 shows a planar integrated optical splitter 90, which can functionas the 1×N optical beam splitter 80 of FIG. 5 (for N=5). The opticalsplitter 90 has an input hole 92. The hole 92 diffracts each receivedsource pulse into five mutually coherent pulses, which are directedalong different directions. Each mutually coherent pulse is collected bya separate optical waveguide 94-99, which carries the pulse to anoptical conduit P₁-P₅. The optical waveguides P₁-P₅ can be continuationsof the waveguides 94-99 or optical fibers of various lengths.

FIG. 7 is a flow chart showing a method 102 of transmitting an opticalpulse in a transmission system, e.g., the system 50 of FIGS. 2 and 5. Toproduce a source pulse, the programmable shutter 77 chops a coherentsource beam from the laser 74 into pulses (step 104). The 1×N opticalbeam splitter 80 divides the source pulse into N coherent pulsesdirected towards the different optical waveguides P₁-P_(N) (step 106).Each of the N coherent pulses travels through one of the waveguidesP₁-P_(N) to produce a pulse with a different delay (step 108). Eachwaveguide P₁-P_(N) delays pulses therein by a time proportional to thelength of the waveguide's optical path. Next, the inverted 1×N beamsplitter 82 recombines the delayed pulses to produce a series ofmutually coherent pulses (step 110). Each successive pulse of the serieshas a different time delay. The inverted 1×N beam splitter 82 acts as anoptical combiner, which transmits the series of coherent pulses througha transmission medium (step 112). The transmission medium may be theoptical fiber 52 of FIG. 2, free space, or another medium fortransmitting optical signals. The receiver 65 receives mutually coherentbroadened pulses at the distant receiving point 71 (step 114). Due tointerference, the received pulses form an intensity pattern 70 that isnarrower than any single one of the received pulses at the receiver 65.Generally, dividing the source pulse into several mutually coherent andtemporally spaced pulses at step 104 produces a narrower intensitypattern at the receiver 65 in step 114.

FIG. 8 shows one selection for a series 120 of mutually coherent pulsesproduced by the pulse splitter 79. The N-th pulse is delayed withrespect to the first pulse 121 by (N)^(½) times the delay of the secondpulse 122. This non-uniform spacing of pulses produces, in the receiver65, an interference pattern with an enhanced central maximum 67. Thetemporal spacing of the pulses produces an enhanced central maximum in amanner similar to the manner in which the non-uniform spatial spacing ofthe rings 51 of the zone plate 46 of FIG. 4B produces the focused spot49.

If delay times of the pulses with respect to the first pulse of theseries have the more general form t(N^(D)+C)^(E), pulse compression alsooccurs at the receiver 65. Here, “t” is between about 10⁻³ and 10⁺⁵times the source pulse's coherence time, i.e., the time over which thephase of the source pulse is correlated. The numbers C, D, and E are allbetween about −10 and +10.

FIG. 9 shows the packet 124, which is received at point 71 in FIG. 2,for the non-uniform pulse spacings of FIG. 8. The packet 124 has asingle central maxima 126 and very small or absent secondary maxima 127.The central maxima 126 is much narrower than the envelope 128, which theoriginal source pulse (not shown) would have produced after travelingdown the fiber 52. Thus, this non-uniform spacing of pulses in theseries 120 produces a real pulse compression at the distant point 71 ofFIG. 2.

FIGS. 10-12D show alternate embodiments for the pulse splitter 79 usedby the transmitter 72 of FIG. 5.

FIG. 10 shows a second pulse splitter 130, which uses a single 1×4 fibercoupler 136 to produce the series of four delayed coherent pulses 56-59shown in FIG. 2. An optical circulator 132 transmits the source pulse toan arm 134 connected to the 1×4 fiber coupler 136. The four outputs ofthe fiber coupler 136 couple to optical waveguides P₁-P₄. Each waveguideP₁-P₄ has a reflector 138-141 attached to its free end to reflect anypulse incident thereupon. Each pulse receives a time delay equal totwice the optical length of the waveguide P₁-P₄. The delayed series ofpulse return along the arm 134 and are directed by the circulator to theoptical fiber 52.

FIGS. 11A-11B illustrate pulse splitters 144, 146, which use a series148, 150 of birefringent elements. Some of the birefringent elements ofthe series 148, 150 may be polarization maintaining erbium doped fibers,which are optically pumped to produce gain. In the four element series148, 150, the consecutive elements have thicknesses forming the sequenceL, 2L, 4L, 8L. For an N element series, the elements will have opticallengths L, 2L, 4L, . . . 2^(N)L and will produce a series of N pulses.Adjacent elements 148, 150 have their optical directrixes rotated byabout 45 degrees. Here, the optical directrix is an intrinsic axis of abirefringent medium along which the refractive index is independent ofthe polarization. Finally, the source pulse is polarized at either 45degrees or 0 degrees to the directrix of the last element 149, 151 ofeach series 148, 150. Polarizers 152, 154 filter the output pulses fromthe series 148, 150 to produce the series of the equally-spaced pulses56-59 shown in FIG. 2.

FIGS. 12A-12D illustrate alternate pulse splitters 160-163. Each pulsesplitter 160-163 uses a series 164-167 of birefringent elements. Some ofthe birefringent elements may be polarization maintaining erbium dopedfibers, which are optically pumped to produce gain. All elements, e.g.,the elements 168-171, of each series 164-167 have equal thicknesses.Adjacent elements of each series 164-167 have a fixed rotation anglebetween their optical directrixes.

The rotation angle is equal to 90 degrees divided by the number ofelements in the series. The source pulse is plane polarized. Thepolarization plane of the source pulse makes a tilt angle with respectto the directrix of the first slab 168-171 of the relevant series164-167. The tilt angle equals half of the rotation angle. Polarizers172-175 filter the mutually coherent pulses produced by each series164-167 to produce a single polarization in the equally spaced pulses56-59 shown in FIG. 2.

FIG. 13 illustrates another pulse splittet 180, which uses a series of2×2 fiber couplers 181-184 and pairs of optical fibers 186-188 toproduce the equally spaced pulses 56-59 shown in FIG. 2. The 2×2 fibercouplers 181-184 form cascaded Mach Zehnder interferometers. Each MachZehnder 181-184 interferometer splits each received pulse into twopulses. The two pulses acquire a timing difference of “T” aftertraveling through the two associated output fibers 186-188 of unequallength. The timing differences accumulate, because the pulses travelthrough several stages of the interferometers.

Generally, the pulse splitter 180 may have N stages to produce Nmutually coherent, time-delayed pulses. The pulse splitter 180 also hasa second output 189, which produces a second series of mutuallycoherent, time-delayed pulses.

FIG. 14A shows a first optical transmission system 190, which places thepulse splitter 79 in the transmitter 54. Thus, a series of coherentpulses 191 propagates down the fiber 52 in the system 190. Interferenceamong dispersion broadened pulses produces a compressed pulse 192 at thereceiver 65.

FIG. 14B shows a second optical transmission system 193, which placesthe pulse splitter 79 in the receiver 194. Here, a transmitter 196 sendsa single coherent source pulse through the fiber 52. The source pulse197 becomes a wide pulse 198 due to dispersion. At the receiver 194, thepulse splitter 79 splits the widened source pulse 198 into a series ofwidened pulses, which are time delayed. Since each pulse of the seriesis mutually coherent, an interference pattern 199 is produced in thereceiver 194. The interference pattern 199 has a narrower centralmaximum 200 similar to the maxima 67, 126 of FIGS. 2 and 9.

Referring to FIGS. 14A-14B, the various embodiments may include opticalamplifiers 208 to boost the amplitude of the pulses in the transmissionfiber 52. For example, the amplifiers 208 may use optically pumpederbium fibers to augment the intensity of received optical pulses.

FIG. 14C shows a transmission system 207, which positions opticalregenerators 252 and 254 at convenient intermediate locations along thefiber 52. Each regenerator 252, 254 includes an amplifier 208, a filter209, and a pulse splitter 79. The amplifiers 208 augment the intensitiesof the received optical pulses. The filters 209 are narrow bandpassoptical filters or nonlinear optical loop mirrors configured to removesecondary maxima from the interference patterns of the received pulses.The pulse splitters 79 form series of mutually coherent pulses from thereceived pulses to counteract broadening produced in the followingsegment of the optical fiber 52.

Referring again to FIG. 2, the transmitter 54 of some embodiments uses aspecial laser configured to produce the series of mutually coherentdelayed pulses 56-59. Such lasers produce a series of mutually coherentoptical pulses 56-59 in response to either a single control electricalpulse or a series of such electrical pulses when operated in knownconfigurations. The laser transmits the series directly into the opticalfiber 52 without sending the pulses through other devices to enhanceinter-pulse spacings. The inter-pulse spacings cause pulse compressionthrough interference at the point 71 within the receiver as shown inFIG. 2.

FIG. 15A shows a transmitter 210 that interleaves mutually incoherentoptical pulses to reduce interference between pulses for differentdigital data bits. A generator 212 produces electrical signals for thesequence of digital data bits to be transmitted across the optical fiber52. Through lines 214-216, the generator 212 transmits the electricalsignals to lasers 73-75. The lasers 73-75 produce optical source pulses.The electrical signals from the generator 212 control individual opticalshutters (not shown) that chop the output beams of each laser 73-75 intothe optical source pulses. An optical combiner 222 receives the opticalsource pulses through fibers 218-220 and transmits the source pulses A,B, and C to a fiber 224.

The generator 212 temporally interleaves the transmission of sourcepulses from the first, second, and third lasers 73-75. The interleavingensures that the same laser 73-75 does not produce consecutive pulses A,B, C for consecutive data bits. For example, the generator 212 may sendthe signal for the first data bit to the first laser, the signal for thesecond data bit to the second laser 74, and the signal for the thirddata bit to the third laser 75. Since none of the lasers 73-75 receivesconsecutive electric signals, each consecutive optical source pulse A,B, and C come from different ones of the lasers 73-75.

The fiber 224 transmits the pulses A, B, and C to the pulse splitter 79of FIG. 5. For each received pulse A, B, and C, the pulse splitter 79produces a series of delayed mutually coherent pulses A′, B′, and C′ asdescribed in FIGS. 2 and 5. The different sequences of delayed pulsesA′, B′, C′ are interleaved in time.

As dispersion broadening occurs each pulse A′, B′, C′ of the differentseries can physically overlap. For example, the series of A′ pulsesassociated with the original A pulse may overlap with the series of B′pulses associated with the original B pulse. The transmitter 210 reducesundesired interference between the different series of pulse A′, B′, C′by making the original pulses A, B, C mutually incoherent. The pulses A,B, C are mutually incoherent, because each consecutive pulse A, B, C isproduced by a different one of the lasers 73-75.

FIG. 15B shows a regenerator or receiver 230, which enhances the centraland/or removes the secondary maxima 234 and 236 from the interferencepattern 238 produced by the pulses 56-59 of FIG. 2. The receiver 230amplifies the received optical signal with an optical amplifier 240 andthen sends the amplified signal to a nonlinear optical loop mirror(NOLM) 242. The NOLM includes a 4-port optical coupler 244 in which twoports are coupled by an optical fiber loop 246. The fiber loop 246includes a standard single mode fiber (SMF), e.g., the model SMF-28fiber produced by Corning, Inc, of Corning N.Y. The SMF connects inseries to a dispersion shifting fiber (DSF).

The NOLM 242 is a light intensity discriminator, which filters outoptical signals having intensities below a preset threshold. By matchingthe amplifier 240 to the NOLM 242, the NOLM 242 filters out thesecondary maxima 234 and 236 to produce an output signal consisting of asingle narrow peak 247. Instead of the NOLM 242, some other. embodimentsuse a different narrow bandpass optical filter to remove the secondarymaxima 234 and 236.

The width of the central maxima 232 can also be modified by varying thenumber of mutually coherent pulses in the series, e.g., the series ofpulses 56-59 of FIG. 2. Increasing the number of mutually coherentpulses in the series generally narrows the central maximum 232. If thenumber of mutually coherent pulses is large enough, the central maximum232 can be as narrow as the original source pulse 249 from the shutter77.

Some embodiments shift the position of the central maximum 232 in theinterference pattern 238 through the time delays between the mutuallycoherent pulses 56-59. Shifting the position of the central maximum 232in the interference pattern 238 changes its wavelength and gives theoutput pulse 247 a wavelength shifted with respect to that of the sourcepulse 249. Thus, the system 252 of FIG. 15B can act as a “frequencyconverter” based on dispersion.

FIGS. 16A-16C show that the pulse splitter 79 can be used to generatehigh frequency optical timing signals from low frequency optical timingpulses. In FIG. 16A a series of low frequency pulses 261 is transmittedthrough into an optical fiber 262. The pulses travel through a length ofdispersive fiber, e.g., a long length of fiber rolled up on a fiberwheel 263, and produce broad pulses 264. In FIG. 16B, the pulse splitter79 is inserted into the fiber 262 to generate interference, whichmodulates pattern 266 on the broad pulse 264. FIG. 16C shows that thespacing “d” between the peaks of the modulated pattern 266 provides anoptical timing signal having a higher frequency than the initial pulserate T⁻¹.

The spacing “d” between the output pulses 267-268 is proportional to thedispersion and inversely proportional to the delay introduced betweenmutually coherent pulses by the pulse splitter 79. The frequency of thefinal timing pulses 267-268 of FIG. 16C can be adjusted to an integralmultiple original frequency of the pulses of FIG. 16A. By adjusting thelength of fiber on the roll 263 and/or the delays produced by the pulsesplitter 79, one can generate final frequencies, which are integralmultiples of the original frequency.

Finally, the techniques and devices of FIGS. 2 and 5 can reduce opticalpulse broadening when pulses are transmitted in other media, e.g., freespace. In free space transmission, the optical fiber 52 is replaced byan atmospheric transmission link. The atmosphere also generateschromatic dispersion and scattering in pulses, which also lead to pulsebroadening. Splitting the original pulses into a temporal series ofmutually coherent pulses, as shown in FIG. 2, can reduce pulsebroadening in atmospheric transmission systems.

For atmospheric transmission systems, the pulse splitter 79 transmitsthe series of mutually coherent pulses 56-59 to the “atmospherictransmission link”. The series of pulses broaden and interfere duringatmospheric transmission. But, the receiver 65 receives the narrowerpattern 64 from the atmosphere due to interference.

Other embodiments are within the scope of the following claims.

What is claimed is:
 1. A method for transmitting a signal pulse in adispersive transmission media, said method comprising: for the signalpulse, producing a series of temporally spaced, substantiallynon-overlapping, mutually coherent optical pulses; and transmitting theseries of optical pulses through the dispersive transmission media, theoptical pulses of the series of optical pulses having a temporal spacingselected such that after traveling a distance along the transmissionmedium, the temporally spaced, substantially non-overlapping opticalpulses broaden and overlap and as a consequence interfere to form aninterference pattern having an interference peak that is narrower thanthe broadened optical pulses.
 2. The method of claim 1, wherein thetransmitting sends the series of optical pulses through an opticalfiber.
 3. The method of claim 1, wherein the signal pulse is in the formof coherent optical pulse and wherein the producing of the series ofcoherent optical pulses comprises separating the coherent optical pulseinto a plurality of mutually coherent optical pulses and wherein theproducing of the series of coherent optical pulses involves producingsaid series of temporally spaced, substantially non-overlapping,coherent optical pulses from said plurality of mutually coherent pulses.4. The method of claim 3, wherein the producing further comprises:sending each of said plurality of optical pulses through a separatewaveguide to produce a delayed pulse, each delayed pulse having adifferent delay; and recombining the delayed pulses to produce theseries temporally spaced, substantially non-overlapping, coherentoptical pulses.
 5. The method of claim 4, wherein the time delays areadapted to remove secondary interference maxima between the series ofpulses at the distant region.
 6. The method of claim 4, wherein thesuccessive pulses of the series are approximately equally spaced.
 7. Themethod of claim 5, wherein each successive pulse of the series has adelay of approximately t_(R)(N^(D)+C)^(E) with respect to a precedingpulse, the number N being a serial order of the preceding pulse, andwherein t_(R) is a positive real number, and D, C and E are realnumbers.
 8. The method of claim 7, wherein t_(R) is between about 10⁻³and 10⁵ times a coherence time of the source pulse, and C, D, and E arebetween about −10 and +10.
 9. The method of claim 1, wherein thetransmitting sends the series of pulses through free space.
 10. Anapparatus for transmitting an optical signal in an optical fiber, saidapparatus comprising: a beam splitter to split a coherent optical pulseinto a plurality of coherent optical pulses; a plurality of opticalwaveguides, each waveguide located to receive a corresponding differentone of the plurality of coherent light pulses and to produce atemporally delayed optical pulse, each waveguide being an optical pathwith a different optical length; and a combiner located to receive thetemporally delayed optical pulse from each of said plurality ofwaveguides and to redirect the received optical pulses into the opticalfiber as a sequence of optical pulses, wherein the plurality of opticalwaveguides are selected so that the sequence of optical pulses is asequence of substantially non-overlapping, temporally spaced, coherentoptical pulses that interfere to form an interference pattern having aninterference peak that is narrower than the broadened optical pulses.11. The apparatus of claim 10, wherein the waveguides are optical fibershaving different lengths.
 12. The apparatus of claim 10 furthercomprising: a source of polarized coherent pulses; and wherein the beamsplitter and the plurality of optical waveguides comprise a series ofbirefringent elements located to receive each polarized pulse from thesource, the series of birefringent elements to produce a series ofequally spaced pulses from each polarized pulse; and a polarizer locatedto project the series of pulses in a selected direction.
 13. Anapparatus for transmitting an optical pulse, said apparatus comprising:an optical fiber; a source of coherent optical pulses; a pulse splitterto split each of the coherent optical pulse into a sequence of mutuallycoherent optical pulses, said pulse splitter including a plurality ofoptical delay elements, each delay element producing a different delayin a corresponding one of the mutually coherent optical pulses, thedelays of the plurality of optical delay elements selected so that saidsequence of mutually coherent optical pulses is a sequence ofsubstantially non-overlapping, temporally spaced, coherent opticalpulses; and an optical circulator located to send the pulses from thesource to the pulse splitter and to send the sequence of mutuallycoherent pulses from the pulse splitter to the fiber that interfere toform an interference pattern having an interference peak that isnarrower than the broadened optical pulses.
 14. The apparatus of claim13, wherein the plurality of optical delay elements is a plurality ofoptical waveguides and wherein the pulse splitter further comprises: a1×N beam splitter having an input located to receive light from thecirculator and having N outputs, wherein each of said outputs isconnected to a first end of a corresponding different one of the opticalwaveguides; and a plurality of reflectors, each reflector attached to asecond end of a corresponding different one of the optical waveguides.15. A transmission system for optical signals, said system comprising: atransmitter to produce source optical pulses; an optical pulse splitterto produce from each of the source optical pulses a series of mutuallycoherent optical pulses, said pulse splitter including a plurality ofoptical delay elements, each delay element producing a different delayin a corresponding one of the mutually coherent optical pulses, thedelays of the plurality of optical delay elements selected so that saidseries of mutually coherent optical pulses is a series of substantiallynon-overlapping, temporally spaced, mutually coherent optical pulsesthat interfere to form an interference pattern having an interferencepeak that is narrower than the broadened optical pulses; an opticalfiber connecting the transmitter to the optical splitter; a receiver foroptical signals; and an optical transmission channel connecting thesplitter to the receiver.
 16. The system of claim 15, wherein thesplitter is adapted to produce pulse compression at the receiver. 17.The system of claim 15, wherein the plurality of optical delay elementsis plurality of optical waveguides, each of which is characterized by adifferent optical path length, and wherein the pulse splitter comprises:a beam splitter to split each source pulse into a plurality of opticalpulses, wherein each of said plurality of optical waveguides is locatedto receive a corresponding different one of the plurality of opticalpulses; and an optical combiner located to receive output from each ofsaid optical waveguides and to redirect the output received from theoptical waveguides into an optical fiber.
 18. The system of claim 17,wherein the optical waveguides are optical fibers having differentlengths.
 19. The system of claim 15, wherein the pulse splittercomprises: a series of birefringent elements located to receive thesource pulses, the series birefringent elements to produce a sequence ofequally spaced optical pulses from each source pulse; and a polarizerlocated to project the sequence of optical pulses into a selecteddirection as the series of mutually coherent optical pulses.
 20. Thesystem of claim 15, wherein the pulse splitter comprises: a pulsesplitter element to split each of the source pulses into a sequence ofmutually coherent optical pulses; and an optical circulator located tothe send optical pulses from the source to the pulse splitter elementand to send the mutually coherent optical pulses from the pulse splitterelement to the optical transmission channel.
 21. The system of claim 20,wherein the plurality of optical delay elements is plurality of opticalwaveguides, each of which is characterized by a different optical pathlength, and wherein the pulse splitter element comprises: a 1×N beamsplitter having an input located to receive light from the circulator,wherein each of said plurality of optical waveguides has a first endcoupled to a corresponding different one of the outputs of the 1×N beamsplitter; and a plurality of reflectors, each of which is coupled to asecond end of a corresponding different one of the optical waveguides.22. The system of claim 15, further comprising a regenerator having aninput coupled to the pulse splitter by the optical transmission channel;and a second optical transmission channel coupling an output of theregenerator and to the receiver, the regenerator including an opticalamplifier and a second pulse splitter to regenerate received pulses. 23.The system of claim 22, wherein the regenerator includes one of anoptical bandpass filter and an intensity discriminator to removesecondary maxima from interference patterns.
 24. The system of claim 15,wherein the receiver includes one of an optical bandpass filter and anintensity discriminator to remove secondary maxima from interferencepatterns.
 25. A method for producing optical timing pulses, said methodcomprising: transmitting a sequence of original optical timing pulsesthrough a dispersive transmission medium to produce a second sequence ofoptical pulses that are broadened as a result of passing through thedispersive medium, wherein the original optical timing pulses of thefirst sequence are characterized by a first pulse width and thebroadened pulses of the second sequence are characterized by a secondpulse width that is larger than the first pulse width; and for eachoptical pulse of the second sequence, (a) generating a plurality oftemporally delayed, mutually coherent optical pulses, each member of theplurality of optical pulses having a different delay, and (b) combiningthe plurality of temporally delayed optical pulses so that theyinterfere with each other to form an interference pattern characterizedby equally spaced apart intensity peaks, the intensity peaks of saidinterference pattern being a timing signal having higher frequency thanthe frequency of the original optical timing pulses.
 26. The method ofclaim 25, wherein the higher frequency is an integer multiple of thetemporal frequency of the sequence of original timing pulses.
 27. Amethod for producing optical timing pulses, the method comprising:receiving a sequence of original optical timing pulses; for each opticaltiming pulse of the sequence, generating a series of temporally spaced,substantially non-overlapping, mutually coherent optical pulses; andtransmitting the series of optical pulses through a dispersive opticaltransmission medium, the optical pulses of the series of optical pulseshaving a temporal spacing selected such that after traveling a distancealong the transmission medium, the temporally spaced, substantiallynon-overlapping optical pulses broaden and overlap and as a consequenceinterfere to form an interference pattern characterized by equallyspaced apart intensity peaks, the intensity peaks of said interferencepattern being a timing signal having higher frequency than the frequencyof the original optical timing pulses.
 28. The method of claim 27,wherein the generating comprises: for each optical timing pulse of thesequence, (a) producing a plurality of mutually coherent optical pulses;(b) sending each of said plurality of optical pulses through a separatewaveguide to produce a delayed optical pulse, each delayed pulse havinga different delay; and (c) recombining the delayed pulses to produce theseries of optical pulses.